An ion source has been assembled at Rice University which uses the principle of optical pumping to produce polarized He ions for extraction and acceleration. The design of an experiment to measure the degree of polarization of the ions is described. 3He+ ions are accelerated to an energy of approximately 300 keV and produce protons through the 2H(3 He,p) 4He reaction on striking a deuterated target. The polarization of the outgoing protons, measured through the left-right asymmetry of their elastic scattering in a helium-filled polarimeter, provides the information necessary to deduce the nuclear spin polarization of the 3He ions. The polarimeter and its associated equipment are described. The results of tests of the polarimeter system with alpha particles and with unpolarized protons are given.